Phosphoramidates, e.g., (RO).sub.2 P(O)N(R).sub.2 wherein R is alkyl, are known compounds. These compounds have been prepared by reacting primary and secondary amines with phosphoric halides, substituted phosphoric halides, and organic halophosphates. These reactions are basically carried out in inert solvents such as ethers at low temperatures and the products are isolated after the separation of the amine salt by-product. The unsubstituted phosphoramidate (RO).sub.2 P(O)NH.sub.2 and particularly the diaryl phosphoramidate is a known compound (U.S. Pat. No. 3,240,729). This compound is an effective reactive flame retardant for use in preparing polyurethanes. These compounds are, however, unstable in caustic solution thereby rendering them undesirable for use in rayon spinning solutions. This problem has been unexpectedly overcome by forming the O,O-diaryl N,N-dialkyl phosphoramidate. This is claimed in the co-pending application of Eilers and Toy filed concurrent herewith.
In attempting to prepare O,O-diaryl N,N-dialkyl phosphoramidates by known routes numerous problems were encountered which were of significance to the commercial development of an economically competitive product. One of the processes investigated involves the reaction between a diaryl phosphorohalidate and a 100% excess of dialkyl amine. While quantitative yields of high purity in the reaction flask can be obtained, quantitative isolation of the final phosphoramidate product is not easily attained. Also, problems exist in the preparation of the diaryl phosphorohalidate intermediate.
Firstly, the diaryl phosphorohalidate, such as diphenyl phosphorochloridate (used hereinafter as exemplary) in alternatively prepared by chlorinating diphenyl hydrogen phosphonate or by the Todd process, the details of which are set forth more fully hereinafter. In chlorinating the phosphonate, hydrogen chloride is evolved as a by-product. The hydrogen chloride is soluble in the diphenyl phosphorochloridate making its removal difficult. Isolation of the pure compound without combination is, therefore, difficult. The economics of the final product require inexpensive starting materials. Low yields of the starting material raise its cost and tend to destroy the economics of the process.
A further problem is encountered when the phosphorochloridate is reacted with a secondary dialkyl amine. As a by-product of the reaction there is formed one mole of dialkyl amine hydrochloride per mole of product. This material flocculates and is extremely difficult to separate from the final product. Once the floc is separated, it is difficult and expensive to dispose of. For instance, dimethyl amine is an expensive reactant, and one extra mole of this amine is required for each mole of product. The loss of the amine in the by-product is economically unattractive. If the floc comes into contact with caustic, the dimethyl amine can be regenerated. Since the dimethyl amine is toxic, waste disposal techniques must, therefore, be carefully controlled. Regeneration and recycling of the amine is uneconomical and impractical.
The Todd process involves the reaction of the diaryl hydrogen phosphonate with a 100% excess of dialkyl amine in the presence of an excess of perchlorinated alkane, such as carbon tetrachloride. This process is even more costly than above and incurs basically the same disadvantages as the process discussed above.
It has now been found that these difficulties can be overcome and O,O-diaryl N,N-dialkyl phosphoramidates can be easily prepared by the new process of the present invention. The compounds as prepared by this new process find use as flame retardant agents in polymer compositions.